The present invention relates generally to wireless communication systems and methods, and more particularly to mode selection for encoding data for transmission in a wireless communication channel based on statistical parameters.
Wireless communication systems serving stationary and mobile wireless subscribers are rapidly gaining popularity. Numerous system layouts and communications protocols have been developed to provide coverage in such wireless communication systems.
Wireless communications channels between transmit and receive devices are inherently variable and their quality fluctuates. Specifically, the quality parameters of such communications channels vary in time. Under good conditions wireless channels exhibit good communication parameters, e.g., large data capacity, high signal quality, high spectral efficiency and throughput. At these times significant amounts of data can be transmitted via the channel reliably. However, as the channel changes in time, the communication parameters also change. Under altered conditions former data rates, coding techniques and data formats may no longer be feasible. For example, when channel performance is degraded the transmitted data may experience excessive corruption yielding unacceptable communication parameters. For instance, transmitted data can exhibit excessive bit-error rates or packet error rates. The degradation of the channel can be due to a multitude of factors such as general noise in the channel, multi-path fading, loss of line-of-sight path, excessive Co-Channel Interference (CCI) and other factors.
In mobile systems, signal degradation and corruption is chiefly due to interference from other cellular users within or near a given cell and multipath fading, in which the received amplitude and phase of a signal varies over time. The fading rate can reach as much as 200 Hz for a mobile user traveling at 60 mph at PCS frequencies of about 1.9 GHz. In such environments, the problem is to cleanly extract the signal of the user being tracked from the collection of received noise, CCI, and desired signal portions.
In Fixed Wireless Access (FWA) systems, e.g., where the receiver remains stationary, signal fading rate is less than in mobile systems. In this case, the channel coherence time or the time during which the channel estimate remains stable is longer since the receiver does not move.
Prior art wireless systems have employed adaptive modulation of the transmitted signals with the use of feedback from the receiver as well as adaptive coding and receiver feedback to adapt data transmission to changing channel conditions. Such adaptive modulation is applied to Single Input Single Output (SISO) and Multiple Input Multiple Output (MIMO) systems, e.g., systems with antenna arrays at the transmit and receive ends.
In both SISO and MIMO systems, however, the fundamental problem of efficient choice of the mode to be applied to the transmitted data remains. For general prior art on the subject the reader is referred to A. J. Goldsmith et al., xe2x80x9cVariable-rate variable power MQAM for fading channelsxe2x80x9d, IEEE Transactions of Communications, Vol. 45, No. 10, October 1997, pp. 1218-1230; P. Schramm et al., xe2x80x9cRadio Interface of EDGE, a proposal for enhanced data rates in existing digital cellular systemsxe2x80x9d, Proceedings IEEE 48th Vehicular Technology Conference (VTC"" 1998), pp. 1064-1068; and Van Noblen et al., xe2x80x9cAn adaptive link protocol with enhanced data rates for GSM evolutionxe2x80x9d, IEEE Personal Communications, February 1999, pp. 54-63.
U.S. Pat. No. 6,044,485 to Dent et al. teaches a transmission method and system which adapts the coding of data based on channel characteristics. The channel characteristics are obtained either from a channel estimation circuit or from an error feedback signal. In particular, when the signal-to-noise (SNR) characteristic is chosen as an indicator of the state of the channel, then a different coding is applied to the data being transmitted for high and low SNR states of the channel. In addition, the encoding also employs information derived from the cyclic redundancy check (CRC).
The method taught by Dent varies the coding rate only and not the modulation rate. This method does not permit one to select rapidly and efficiently from a large number of encoding modes to adapt to varying channel conditions.
U.S. Pat. No. 5,559,810 to Gilbert et al. teaches a communication system using data reception history for selecting a modulation technique from among a plurality of modulation techniques to thus optimize the use of communication resources. At least one block of data is transmitted with a particular modulation technique and a data reception history is maintained to indicate transmission errors, e.g., by keeping a value of how many blocks had errors. The data reception history is updated and used to determine an estimate of transmission signal quality for each modulation technique. This value is then used in selecting the particular modulation technique.
The system and method taught by Gilbert rely on tracking errors in the reception of entire blocks of data. In fact, signal quality statistics, e.g., signal-to-interference and noise ratio (SINR) are derived from the error numbers for entire blocks of data, which requires a significant amount of time. Thus, this method does not permit one to select rapidly and efficiently from a large number of encoding modes to adapt to varying channel conditions, especially in the event of rapid fades as encountered in mobile wireless systems.
It would be an advance to provide a mode selection technique which allows the system to rapidly and efficiently select the appropriate mode for encoding data in a quickly changing channel. It is important that such technique be efficient in all wireless systems, including Multiple Input Multiple Output (MIMO), Multiple Input Single Output (MISO), Single Input Single Output (SISO) and Single Input Multiple Output (SIMO) systems as well as systems using multiple carrier frequencies, e.g., OFDM systems.
The present invention provides a method for selecting a mode for encoding data for transmission in a wireless communication channel between a transmit unit and a receive unit. The data is first encoded in accordance with an initial mode and transmitted from the transmit unit to the receive unit. One or more quality parameters are sampled in the data received by the receive unit. Then, a first-order statistical parameter and a second-order statistical parameter of the quality parameter are computed and used for selecting a subsequent mode for encoding the data.
The one or more quality parameters can include a short-term quality parameter or several short-term quality parameters and be selected among parameters such as signal-to-interference and noise ratio (SINR), signal-to-noise ratio (SNR) and power level. Conveniently, a first sampling time or window is set during which the short-term quality parameter is sampled. In one embodiment, the length of the first sampling window is based on a coherence time of the wireless communication channel. In another embodiment, the duration of the first sampling window is based on a delay time required to apply the subsequent mode to encode the data at the transmit unit. In yet another embodiment, the second-order statistical parameter is a variance of the short-term quality parameter and the length of the first sampling window is selected on the order of the variance computation time.
The one or more quality parameters can also include a long-term quality parameter or several long-term quality parameters. The long-term quality parameter can be an error rate of the data, such as a bit error rate (BER) or a packet error rate (PER) at the receive unit. Again, it is convenient to set a second sampling time or window during which the long-term quality parameter is sampled. In one embodiment, the first-order statistical parameter is a mean of the long-term quality parameter and the length of the second sampling window is set on the order of the-mean computation time. In another embodiment, the length of the second sampling window is set on the order of an error rate computation time.
In many instances, it is convenient when the first-order statistical parameter is a mean of the quality parameter and the second-order statistical parameter is a variance of the quality parameter. The variance can include two variance types: a temporal variance and a frequency variance. The latter is useful in systems employing multiple frequencies for transmitting the data. Specifically, it is particularly convenient to compute both temporal and frequency variances when the data is transmitted in accordance with a multi-carrier scheme.
The initial mode for encoding the data can be selected from a set of modes. The set of modes can be made up of a number of modes which are likely to work based on a preliminary analysis of the channel. The set of modes can be organized in accordance with the at least one quality parameter whose first-order and second-order statistics are used in subsequent mode selection.
Conveniently, the subsequent mode is communicated to the transmit unit and applied to the data to maximize a communication parameter in the channel. For example, the subsequent mode can maximize data capacity, signal quality, spectral efficiency or throughput of the channel or any other communication parameter or parameters as desired.
The method of the invention can be used in Multiple Input Multiple Output (MIMO), Multiple Input Single Output (MISO), Single Input Single Output (SISO) and Single Input Multiple Output (SIMO) communication systems, e.g., receive and transmit units equipped with multiple antennas. Furthermore the method can be used: in communication systems employing various transmission protocols including OFDMA, FDMA, CDMA, TDMA.
The method of invention can also be used for selecting the mode from a set of modes and adjusting the selection. For this purpose data encoded in an initial mode selected from the set of modes is received by the receive unit. The short-term quality parameter is then sampled to determine a statistical parameter of the short-term quality parameter. Of course, the statistical parameter can include any combination of first-order and second-order statistical parameters. The subsequent mode is selected based on the short-term statistical parameter. In addition, the long-term quality parameter of the data received by the receive unit is also sampled. The subsequent mode selected based on the short-term statistical parameter is then adjusted based on the long-term quality parameter.
The set of modes can be arranged in any suitable manner, e.g., it can be arranged in a lookup table and ordered by the short-term quality parameter and specifically the first-order and/or second-order statistics of the short-term quality parameter for easy selection. In fact, the lookup table can be modified based on the short-term quality parameter.
The invention also encompasses a system for assigning a subsequent mode for encoding data. The system has a transmit unit equipped with a transmit processing block for encoding the data in a mode. A receive unit is provided for receiving the data transmitted from the transmit unit. The receive unit has a statistics computation block for sampling at least one quality parameter of the received data and computing the first-order and second-order statistical parameters of the data. The receive unit also has a mode selection block for assigning the subsequent mode based on the first-order and second-order statistical parameters.
Conveniently, the system has at least one database containing the set of modes from which the mode, e.g., the initial mode, and the subsequent mode are selected. In one case, the receive has a first database containing the modes and the transmit unit has a second such database.
The system also has a feedback mechanism for communicating the subsequent mode from the receive unit to the transmit unit. This feedback mechanism can be a separate mechanism or comprise the time-division duplexing (TDD) mechanism.
A detailed description of the invention and the preferred and alternative embodiments is presented below in reference to the attached drawing figures.